Spray drying is the transformation of a feed material from a fluid state into a dried particulate form by spraying the feed material into a hot drying medium. It is a one-step, continuous particle-processing operation involving drying. Background information relative to the field in which this invention finds application is provided in Spray Drying Handbook, third edition, K. Masters (John Wiley and Sons, New York).
The invention is applicable to spray drying, spray evaporation and spray reaction operations and processes which usually involve the introduction of a liquid feed material in the form of a solution or a suspension to an atomizing device which sprays small particles of the liquid feed material into a drying chamber where the droplets are dried or reacted and the resulting desired particulate material is collected and removed in the form of a powder. A rotary atomizing wheel which provides a continuous spray of the liquid droplets is often used at the top of a tower-type drying chamber. In addition to the droplets of liquid feed material, a processing gas in the form of a temperature controlled gaseous medium is also introduced to the tower-type drying chamber to evaporate the moisture from the liquid droplets and thus provide the desired particulate material.
Drying chambers of this type utilizing a rotary atomizer wheel and heated air introduced at the top of a drying chamber have been widely used to dry consumable food products, such as milk, to produce chemical products such as kaolin clay, titanium dioxide and calcium carbonate, and for processing waste effluents. Such processes are critically dependent upon the particle size of the material being dried, the temperature of the drying medium and the timely effective contact of each particle with the drying medium.
Rotary atomizer wheels or spray nozzles are generally used for spraying the feed material into the hot gas medium.
Various types of heating devices have been utilized to provide the hot gaseous medium. Gas heaters of the direct and indirect type have been utilized as well as steam, fuel oil, heat transfer fluids and electricity. Selection of a heat source for a particular process depends upon the product being spray dried as well as the availability, suitability and cost of the energy required in order to heat the gaseous medium. Open oil and gas fired burners may be utilized where products can withstand the high temperature generated by such heat sources as well as contact with the resulting products of combustion. Indirect heater utilizing electrical heating elements, gas or fuel oil burners may also be utilized to provide a hot gaseous medium when it is necessary to protect the product being sprayed from contact with products of combustion.
The processing or drying gas medium is delivered from a source to a gas distributor or distribution means from which it is supplied to the interior of the spray drying chamber. The gas is typically heated while moving through the delivery system by means of a burner or other heat source located in the gas delivery system before the gas is delivered to the gas distributor. The heated gas is then conveyed through insulated ducts to the gas distributor for introduction into the drying chamber and subsequent admixture with the feed spray.
A typical process and apparatus for providing a hot drying gas around an atomizing device in a spray drying chamber is disclosed in Pat. Nos. 3,621,902 and 4,227,896. In those processes, the gaseous medium is heated by means of a device located in the gas delivery system remote from the spray drying chamber. Pat. No. 4,227,896 also discloses an efficient gas distributor having a spiral supply duct and a plurality of vanes which provide a gas flow of substantially constant velocity through a conical guide duct.
Pat. No. 3,499,476 discloses a process for the production of particulate solids from a solution or suspension by nozzle spraying the feed material through a flame zone provided by an annular burner at the top of a spray tower. In this process, the gaseous products of combustion are introduced directly into the drying chamber with the liquid particles of feed material.
United Kingdom Patent Specification No. 1,191,032 discloses apparatus providing a preliminary drying zone wherein feed material is sprayed through a nozzle surrounded by a burner and the gases of combustion intermingle with the liquid particles of feed material. A secondary drying agent is delivered around the spray nozzle and gas burner.
Still another spray drying apparatus utilizing an air heater provided in a chamber located directly above a spray drying chamber is disclosed in Pat. No. 4,187,617. In that apparatus air is delivered by means of a fan through a transition duct to a distribution chamber located above the spray drying chamber. An air heater and a profile plate are provided in the distribution chamber together with a perforated diffuser. Cold air is delivered past the profile plate and heater to ensure that the air is uniformly heated and then received through the perforations in the diffuser and ducted to the spray drying chamber. The object of that apparatus is to ensure uniform heating of the air being introduced to the distribution chamber and both uniform temperature and flow of the heated air into the spray drying chamber.
Conventional systems of producing hot processing or drying gas using either direct or indirect heating means in the form of a furnace or combustion device and then ducting or delivering the hot gas from the heating area into the drying chamber by conventional air distributors are known in the art. Such conventional systems are suited for spray drying processes requiring drying gas temperatures which are not in excess of about 550.degree. C. However, such conventional systems are seldom used in spray drying operations requiring drying gas temperatures near or in excess of 1000.degree. C. since conventional systems would require the generation of substantial thermal energy remote from the point of use and extensive use of refractory materials in the delivery system between the heat source and a gas distributor adjacent the spray drying chamber.
One object of the present invention is to provide a gas heater and air distributor for high temperature spray drying systems which requires minimum refractory lining of the heating chamber and hot gas ducting or conveying surfaces. This object is achieved by providing the heat source directly upstream of the point of use in the spray drying chamber thus permitting delivery of a cold gas by conventional conduits to a gas distributor adjacent a spray chamber and then heating the gas supplied from the gas distributor as it is conveyed to the spray chamber. The high temperature, hot drying gas is conveyed directly into the drying chamber which minimizes the refractory material required since the high temperature drying gas is exhausted directly from the area where the gas is heated into the drying chamber.
The present invention is distinctly different from the apparatus disclosed in Pat. No. 3,499,476 and United Kingdom Specification No. 1,191,032 since the heat source and the flame of the present invention do not directly contact the liquid particles of feed material in the drying chamber and the particulate material will not be adversely effected by the products of combustion.
The spray drying apparatus of the present invention is also different from the apparatus disclosed by Pat. No. 4,187,617 since the gas diffuser or distributor in that apparatus is provided between the air heater and the spray nozzle. The apparatus of that patent would also be limited to spray drying processes utilizing drying gas heated to only a moderately high gas temperature on the order of 550.degree. C. because of the diffuser and wire screens downstream of the air heater.
As will be more fully described here below, the apparatus of the present invention is particularly suited for use in high temperature spray drying of materials such as iron oxide pigments, magnesium chloride, and titanium oxide, or industrial waste effluents such as an acid solution of iron sulfate and magnesium sulfate wherein drying gas or hot air having a temperature in the range of 1000.degree. C. or more is required.
Thus, one object of this invention is to provide a gas distributor, gas heater and atomizer for a high temperature drying system.
A further object of this invention is to provide a spray drying apparatus having a drying gas delivery system wherein the maximum amount of gas distribution is accomplished before the gas is heated to the desired temperature, the gas is then heated and conveyed directly to the spraying chamber.
A still further object is to provide a gas heater for receiving gas from a cold gas distributor and having an outlet exiting in the vicinity of the liquid droplets being discharged from an atomizer device.
A further object is to provide a hot processing or drying gas with its maximum temperature attained adjacent an atomizing device.
A still further object is to provide a gas heater for high temperature drying systems comprising a circularly disposed cold air distributor circumscribing and venting past a circumferential array of gas burners located in a circular passageway exiting in the vicinity of an atomizer device.